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This Trojan encrypts data with RSA-4096 -- really?

Antivirus vendors are currently reporting a further instance of so-called ransomware, meaning malware that encrypts files on an infected PC and only decrypts them after receiving payment of ransom. The new extortionist is Win32.Gpcode.ai (Kaspersky ) or Trj/Sinowal (Panda) and it threatens to encrypt the data with RSA-4096. Since RSA with this key length is currently considered to be uncrackable (a 640-bit RSA key was at last factorised in the end of 2005) at first glance it seems as if the victims have no other choice than to submit to the demand. Research by security software manufacturer Prevx suggests that this malware infects the computer via a link to an infected web site in a convincing graphical email offering CV and job search services. The trawled data are apparently stored on a web site hosted by Yahoo in the USA.

You will need at least few years to decrypt these files without our software. All your private information for last 3 months were collected and sent to us.

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To buy our software please contact us at: xxxxxxx@xxxxx.com and provide us your personal code -xxxxxxxxx. After successful purchase we will send your decrypting tool, and your private information will be deleted from our system.

If you will not contact us until 07/15/2007 your private information will be shared and you will lost all your data.

Fortunately, the author of the Trojan is just full of hot air: the data has only been encrypted with a modified version of RC4 instead of with RSA-4096. The specialists from Kaspersky claim that they have developed an decryption routine that they will soon provide as an update to their security programs. Kaspersky advises victims to not pay the ransom.

A similar situation occurred in mid-2005 with the ransom Trojan PGPcoder, which only employed a proprietary, easy to crack method of encryption or encoding instead of the threatened tougher encryption. Kaspersky also provided a decryption routine for this threat in their database.

The infection seems moderately widespread but precise numbers are not yet available as current reports are contradictory. Panda claims to have counted 1,108 affected systems listed on the server on which Sinowal reports successful infiltration. Meanwhile, Prevx has counted a total of 6,317 IP addresses of supposedly infected machines. Prevx further reports that the malicious link is now dead, and that they have made a decryption tool available to assist those who have already been infected, together with a technical paper on the actions of the malware.